mannu1,
20*20 is a pretty good room for a HT setup . I'd say buy a decent avr and start with FS / BS as a startup and then add on remaining one by one . Or else if you have plans to go for HTIB you can look for Onkyo HTIB 5300 ..or higher models.
But a good AVR with FS and sub/centre will be good to start.. Later you can add other components...
Kittu